Color filters are used in a variety of products, such as liquid crystal displays (LCDs), optical filters for cameras, and the like. Color filters can be fabricated by coating a fine region colored with three or more colors to form a colored thin film on a charge coupled device or a transparent substrate. The colored thin film is commonly fabricated by methods such as dyeing, printing, electrophoretic deposition (EPD), and pigment dispersion.
The dyeing method forms a colored film by forming an image with a dyeing agent such as a natural photosensitive resin such as gelatin, an amine-modified polyvinyl alcohol, an amine-modified acryl-based resin, and the like, on a substrate and then dyeing the image with direct dyes. However, the dyeing process can be complex and time consuming, since it should include resist printing whenever a color needs to be changed to form a multicolored thin film on the same substrate. In addition, conventionally used dyes and resins generally have good color vividness and dispersion but poor light fastness, water resistance, and heat resistance, which are very important characteristics. For example, azo and azide compounds can be used as a dye, but they can exhibit deteriorated heat resistance and durability as compared with a pigment.
The printing method forms a colored thin film by printing an ink prepared by dispersing a pigment into a thermally curable or photocurable resin and curing it with heat or light. This method may decrease material costs as compared with other methods, but it can be difficult to form a fine and precise image and obtain a uniform thin film layer.
Korean Patent Laid-Open Publication No. 1995-7003746 discloses a method of making a color filter in an inkjet method. However, this method disperses a dye-type photosensitive resin composition from a nozzle to provide fine and precise color printing and thus it can also has the same problems of deteriorated durability and heat resistance as the dyeing method.
Korean Patent Laid-Open Publication Nos. 1993-7000858 and 1996-0029904 disclose electrophoretic deposition (EPD) using an electric precipitation method. The electrophoretic deposition (EPD) can form a precise color film having excellent heat resistance and light fastness, since it includes a pigment. However, when a finer electrode pattern is needed for a more sophisticated pixel in the future, it may be difficult to use the method to produce a color filter for highly sophisticated applications since the resultant colored thin film may be stained or thicker at both ends due to electrical resistance.
The pigment dispersion method forms a colored film by repeating a series of processes such as coating, exposing to light, developing, and curing a photopolymer composition including a coloring agent on a transparent substrate including a black matrix. This pigment dispersion method can improve heat resistance and durability, which are very important characteristics of a color filter, and can uniformly maintain the thickness of the film. As examples, Korean Patent Laid-Open Publication Nos. 1992-7002502 and 1995-7000359, Korean patent publication No. 1994-0005617, and Korean Patent Laid-Open Publication No. 1995-0011163 disclose a method of making a photosensitive resin composition for a color filter using a pigment dispersion method.
When a photosensitive resin composition for a color filter is prepared according to a pigment dispersion method, it generally includes a binder resin, a photopolymerizable monomer, a photopolymerization initiator, an epoxy resin, a solvent, and other additives. For example, in Japanese Patent Laid-Open Publication Nos. Pyung 7-140654 and 10-254133, the binder resin includes a carboxyl-containing acryl-based copolymer (A).
The color filter can be subjected to many chemical treatments during the manufacturing process. Accordingly, a color photosensitive resin should have a development margin and chemical resistance sufficient to improve yields of the color filter and to maintain a pattern formed under the aforementioned conditions.
Conventional color liquid crystal displays (LCD) are generally manufactured by preparing a color filter substrate for displaying a color image separately from an operating substrate on which a thin film transistor (TFT) array is disposed, and then binding the color filter substrate and the operating substrate together. However, since there can be low arrangement accuracy during the binding step, conventional color liquid crystal displays are required to have a shading layer with a large width. Accordingly, it is difficult to increase the aperture ratio (the ratio of active light-emitting area to total pixel area). In addition, as glass substrates and LCD screens for LCDs become larger, it can takes longer for the liquid crystal composition to cover a front side of the substrates during vacuum injection. A method has been suggested to sharply decrease the time needed for printing a seal material and dripping a liquid crystal for an over-coating, but this method has a problem of sharply deteriorating arrangement accuracy.
A method for forming a color filter on the operating TFT array substrate of a TFT color liquid crystal display (LCD) has also been suggested. Since a color filter substrate is unnecessary, this method has an advantage of simple arrangement and an increased aperture ratio by binding two substrates after fabricating a transparent substrate through sputtering.
However, when a color filter is formed on a TFT array substrate, a pixel electrode is formed on the color filter in a photolithography method by using a general positive photosensitive resin composition. Accordingly, the photosensitive resin film needs to be removed after forming the electrode. In other words, a pixel electrode is formed by forming a transparent electrode layer on color pixels of a color filter, coating a positive resist thereon, patterning it, exposing it to light, and developing it. Then, the photosensitive resin film remaining on the pixel electrode is peeled and removed with a resist stripper. Accordingly, the photosensitive resin composition requires resistance against a resist stripper. Conventional photosensitive resin compositions, however, typically have weak stripper-resistance.
Conventionally, a pixel electrode is fabricated by forming a pixel protective layer having stripper-resistance on a color filter. In addition, a pixel electrode can be fabricated by using a stripper and peeling a positive resist at a low temperature for a longer time without coating a pixel protective layer. However, these methods have problems of deteriorating yield rate and production efficiency, since they require more processes and a longer time.
In order to solve these problems, a radiation-sensitive composition with an expansion rate of less than 5% against the stripper solution to remove a cured layer forming a color layer in a COA method can be used. Further, a color filter can have improved thermal polymerization cross-linking effects by using a multi-functional alicyclic epoxy compound in a thermal polymerization cross-linking agent and a benzophenone-based peroxide as a photo-thermal polymerization initiator. According to this method, a color filter can be cured at a low temperature and for a short time, and can thereby have excellent durability and close contacting (adhesion) properties. However, there is an increased demand for larger screens with higher image quality than can be produced using conventional methods, and thus there is a need for color filters with a higher aperture ratio and higher performance.